The secret of better OLEDs

January 4, 2016

By eeNews Europe

A research team from Karlsruhe Institute of Technology (KIT), the University of St. Andrews, and the OLED expert company Cynora GmbH has explored and quantified the underlying quantum mechanical effect of intersystem crossing in OLEDs. They found that the use of copper can increase energy and cost efficiency of OLEDs.

Regarded as the light source of the future, OLEDs emit light evenly to all directions with brilliant colours. Since they can be manufactured as transparent and flexible devices, they enable innovative application and design options such as flat light sources in windows or rollable display screens.

OLEDs consist of thin layers of organic materials as emitters. If a voltage is applied, electrons are injected by the cathode and defect electrons by the anode. In the emitter, electrons and defect electrons are recombining, forming so called excitons which after a short delay fall back to their original status, releasing energy.

However, excitons can take two different conditions: So-called singulet excitons immediately decay and emit light whereas triplet excitons only emit thermal energy, i.e., heat. To achieve better energy efficiency, designers try to utilise also the triplet excitons for light generation. This is typically done by adding heavy metals such as iridium or platinum. The drawback with this method: these materials are expensive, and their availability is limited. Plus, they are produced in complex processes, and some of them are toxic.

A more cost-effective and environmentally friendly method is using copper complexes as emitter materials. Here, a unique effect called thermally activated delayed fluorescence (TADF) increases the light yield and thus the energetic efficiency. TADF is based on a complicated quantum mechanical phenomenon called intersystem crossing (ISC) that makes it possible to utilise all excitons, triplets as well as singulets, to generate light. With TADF, copper as a luminescent material has an efficiency of 100%.

So far, so known. Recently however, researchers Stefan Bräse and Larissa Bergmann from KIT along with Cynora researchers and scientists from the University of St. Andrews measured the speed of ISC in a highly luminescent copper (I) complex in solid state with thermally activated delayed fluorescence. According to their research report, published in the magazine Science Advances, they succeeded in determining the ISC time constant for the singulet-triplet transition with 27 picoseconds.

The reverse intersystem crossing (from triplet to singulet) is slower, which leads to a TADF of 11.5 microseconds (average). Their measurements help scientists and engineers to better understand the mechanisms that lead to TADF. Thus, they enable OLED designers to develop TADF materials for more energy efficient OLEDs.

For more, see the paper in Science Advances: "Direct observation of intersystem crossing in a thermally activated delayed fluorescence copper complex in the solid state."